<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .outer{
            height: 500px;
            border-bottom: 10px black solid;
            margin: 50px auto;
            /* 开启BFC，防止边距折叠 */
            overflow: hidden;
        }
        .outer div{
            float: left;
            width: 100px;
            height: 100px;
            border-radius: 50%;
            background-color: #bfa;

            animation: ball 1s  ease-in forwards infinite alternate;
        }
        div.box2{
            animation-delay: 0.2s;
        }
        div.box3{
            animation-delay: .3s;
        }
        div.box4{
            animation-delay: .4s;
        }
        div.box5{
            animation-delay: .5s;
        }
        div.box6{
            animation-delay: .6s;
        }
        @keyframes ball{
            from{
                margin-top: 0px;
            }
            
           to{
                margin-top: 400px;
               
            }
           



        }
    </style>
</head>
<body>
    <div class="outer">
    <div class="box1"></div>
    <div class="box2"></div>
    <div class="box3"></div>
    <div class="box4"></div>
    <div class="box5"></div>
    <div class="box6"></div>
    </div>
</body>
</html>